CA1162778A

CA1162778A - Method for dry sensitization of an insulating surface and powder to be used in the method

Info

Publication number: CA1162778A
Application number: CA000349530A
Authority: CA
Inventors: Gunnar Sorensen; Leo G. Svendsen
Original assignee: Neselco AS
Current assignee: Neselco AS
Priority date: 1979-04-11
Filing date: 1980-04-10
Publication date: 1984-02-28
Also published as: IT8067559A0; NO154370C; IT1128410B; IE49192B1; WO1980002222A1; ZA802121B; EP0026211A1; EP0026211B1; IL59807A; IE800732L; JPS56500435A; NO154370B; NO803710L; DK153337B; US4504529A; DE3067809D1; DK153337C; JPH0210593B2; DK150779A

Abstract

ABSTRACT

A method of dry, selective sensitization of the surface of an insulating substrate for electroless deposition of a metal, wherein the transfer takes place by use of a xerographic process for transfer of a dielectric powder containing the metal compound, after which the transferred powder is exposed to treatment that will secure adherence to the surface.

Description

". 1 This invention relates to a method for dry, selective sensitization of the surface of an insulating substrate for electroless deposition of a metal, including transFer of a metal compound to the surface. The invention also relates to a powder to be used in the method.
Within electronics, it has for a long time been desirable to be able to transfer conducting materials in a prescribed pattern (selectively) with good adherence to the surface of an insulating substrate. In particular because the prescribed pattern might have details that do not touch other parts of the pattern, electroless desposition has been used. In this method there can be used for example masking of those parts of the surface which should not be metallized or/and sensitization of the parts which should be metallized. It is well known to use aqueous sensitization solutions and lately (e.g.
as described, in Danish Patent No. 132 801) pure organic solutions have been used. In the Danish Patent No.
132801 an extremely lucid exposition of the function of these solutions, together with advantages of aqueous and organic solvents, has been made.
The method where aqueous solvents are used may be summarized as follows:
(1) Immersing in a Sn solution (sensitizer), rinsing.

(2) Immersing in an activator solution (e.g. a Pd compound)~

1 18277~

rinsing.

(3) Immersing in a metallizing bath.
The method where an organic solvent is used is based upon direct transfer of, for example, Pd ions to the surface of the substrate by evaporating the organic solvent, which might have etched the surface of the substrate.
These methods have the disadvantage that the precision of the metallization is determined by the method used for transfer of the sensitized pattern. As an example can be mentioned serigraphy, where the resolution will be no better than the mesh gauge of the silk screen. Also confluency and diffluency before drying may occur. Another problem in connection with the above methods is that the adherence ln practice does not appear to be very good~ a problem which is well known from non-selective metalllzing.
The solutions of this problem are, for example9 described in SE Patent No. 350 774 (rubbing-in together with slight grinding) 7 GB Patent No. 1 324 653 (ion bombardment), and DE AS No. 1 571 802 (deposition of a binding layer prior to atomization of metal particles).
Transfer of the pattern may also be carried out by a selective desensitization by ultraviolet light, as described in the proceedings of the conference Physics in Industry i976: "New Photoimaging Processes for Industrial Use - a Link between Basic and Applied Research,"
pp. 101-105.

.

~ 1~2778 Still anothe~ method for transfer of a conduct-ing pattern to the surface of a substrate consists in el-ect~ostat~c precipitation of a masklng layer, which is a positive picture of the desired conductive pattern on an existing layer of Cu on the insulating substrate. According to U.S. Patent Nos. 2,966,429 and 2,910,351, the layer is produced ~y confluency during the melting of an electrostatically transferred thin layer of a dielectric powder. After removal by etching of the non-masked Cu layer, the conducting pattern will be left. In U.S.
Patent ~o. 2,910,351, reference is made to a photo-elec-tro-static process involving -the surface of the substrate to be covered by a photo-conducting, insulating layer.

The U.S. Patent No. 2,910,351 also describes that a dielectric powder in an ensuing partial process for electroplating with silver may be mixed with an Ag compound which, after melting-in with the dielectric powder, is reduced to metallic Ag in a bath. In this way, -the conduct-ing pattern obtained in the first partial process is covered by Ag particles, which may then be electroplated with other materials. It should be borne in mind that this partial process implies the presence of a conducting layer, in the description Cu, under the ~ 16~78 transferred and by reduc-tion created Ag particles.
~ he procedure described in the U S patent Nos.
2966429 and 2910351 is time consuming, bwt provides good results when coarse details such as printed circuits wi-th satisfactory adherence are produced. However, bo-th procedures use s-tationary "pat-tern electrodes" for the transfer of -the pattern, which makes it difficult -to alter the conductive pattern. Actually, the transfer only consists in selectively protecting parts of a laminated layer of Cu against being etched away; hence the good adherence may be ascribed to the lamination process.
~ he purpose of -the present invention is to avoid the disadvantages mentioned above and to obtain a transfer of a pattern to a surface of an insulating substrate, which is more flexible (i.e. easy to alter), and which provides a better adherence and greater possibilities of obtaining finer details than do the hitherto known procedures.
~ he present in~ention provides a method of dry, selective sensitization of the surface of an insulating substrate for elec-troless deposition of a metal, wherein the transfer takes place by use of a xerographic process for transfer of a dielec-tric powder containing -the metal compound, a~ter which the transferred powder is exposed to a treatment that will secure adherence to the surface.

':.' ~ ~L6~77~

The powder to be used in the method according to the invention may cornprise a thermoplastic plastic material with an Sn2 compound precipitated on the surface of each powder particle.
Alternatively, the powder to be used in the method according to the invention may comprise a thermoplastic plastic material with an Sn2+ compound distributed in each powder particle.
The powder to be used in the method according to the invention may be such that, when in a molten state, it wets the surface to be sensitized.
The treatment after the transfer of the powder may comprise treating the substrate and powder with an organic solvent to obtain adherence.
Alternatively, adherence between the powder and substrate surface may be obtained by melting of the substrate surface.
The powder to be used in the method according to the invention rnay contain an infrared-absorbing material.
Further the main component of the powder to be used in the method according to the invention may be a material with a large loss-angle at microwave frequencies.
What makes it possible to utilize a procedure as specified in the present invention, and which has not been known hitherto, is that it still may be advantageous --` l 162778 to use a three-step process as that described in the discussion of the prior art~ above, but it is possible to avoid the wet process at the transfer of the Sn2 com-pound ~ the sensitizing part of the process, and the subsequent selective desensi-tizing. The adherence between the Sn2+ compound and the surface of the substrate in the wet process is not overwhelmingly good, but quite surprisingly it has appeared that transfer perforrned according to the invention gives an adherence which is extremely good, i.e. that the Pd and Cu atoms transferred in the following preferred processes have a better fixation to the substrate than it has been possible to obtaln so far.
The xerographic process has appeared to be well-suited for transfer of the sensitizing Sn2 compound as it provides both a suitable coverage of the insulating substrate and a suitable resolutïon, which are normally conflicting factors. Furthermore, many of the plastic qualities used for toning powders in xerographic copying machines are well suited as regards adhesion to a large number of different substrate materials. As an example, reference is made to those mentioned in US Patent No.
2 966 429; furthermore, it will be a simple matter for the man skilled in the art to check the usability of commercially available plastic materials. Good results are obtained with such different materials as poly-7 ~ 162778 acryla-t~s, polyamides, polyvinylchlorides, and phenol resins. As usable substrate materials may be mentioned polyesters, polyacrylates, polyvinyl-chlorides, poly-carbonates, i.e. a great deal of the materials used in practical electronics.
The fixation to the surface of the insulating substrate can be partly obtained by methods which are generally well known, by powders not containing sensitizing metal cornpounds, typically by "burning down". This means that each particle of the powder is heated so that upon melting it spreads on the surface to a degree which depends on the wetting (the surface tension). According to the invention, the same demands are not made to a pore-free confluencing that are made in the procedures that are based on using the layer obtained as a mask prior to etching. This is probably connected with the sensitizing mechanism which, rather than demanding an even surface as a basis for the subsequent electro-less metal deposition, uses the metal-compound particles deposited on the surface as a seed. This means that the demands on the adherence are greater than the demands on the spreading.
With a view to obtaining a local heating of the transferred particles, materials have been added to them which absorb the energy radiati~n, the wavelength of which depends on the application and combination of '' f g 16~778 substrate/powder. In cases where the powder rnaterial is not thermoplastic, it will be expedient to let each powder particle heat the surface of the substrate in its area of contact, and a suitable choice of softening temperature for the substrate will make possible a melting-down in this case. This rnethod is just a special case of the method which consists in heating all the surface to the melting point.
In circumstances where heating is not desirable, fixation can be made by letting the powder on the substrate pass an atmosphere with a suitable vapour pressure of an organic solvent which, in the well-known way, makes the substrate and the particles stick to one another, and subsequently evaporate the solvent.
There are several ways of produ!cing the prepared powders used according to the invention. The methods used so far have all had the purpose of ensuring that each powder particle, at least on its surface, has had a reasonably even distribution of Sn2~ compounds. This can be obtained in two principally different ways:
- either directly by impregnating the surface of each particle, or by producing a solid mass aiming at an even distribution of the sensitizing compound in the plastics material which is subsequently pulverized in the usual way to obtain a suitable size of the particles.
This size is determined by the construction of the .

~ ~2~78 .

xerographic transfer apparatus and is a well-~nown technique.
The invention will be further described with reference to the following illustrative Examples.
Example 1 Production of a sensitizing plastic powder, aiming at an even distribution of tin compounds in the material:
To 10 g of a plastic material is added 0.05 - 0.5 g of SnC12, 2H20.
This mixture can be wet or dry-ground in an aqueous or organic liquid. Then the liquid may be removed by a conventional drying technique, e.g. evaporation, filtration, or spray-drying.
Impregnation of the individual plastic par~ticles with a tin compound may, according to the invention, use as its starting point already existing powdersj but may also be done at the production of the plastic powder.
A more appropriate way of obtaining the desired distribution of tin in the single plastic particle is to do it during the production of the powder. This can, for example, be achieved by atomizer-drying (spray -drying) the plastic-containing solution, which also contains a tin compound. I
Since certain Sn compounds, e.g. SnC12, 2H20, are soluble in organic solvents, it is possible to ~ 162778 o influence the powder particles individually by a solvent, which makes the particles quell and hence absorb the tin compounds which are dissolved in the organic solvent. Evaporation of the solvent creates the desired distribution of SnC12, 2H20.
Example 2 A plastic powder, which quells when an organic solvent is used, is suspended in such a solvént, to which is added a saturated solution of SnC12~ 2H20 in methanol without precipitation of tin compounds. After a certain time of reaction, the solvent can be removed by, e.g., filtering or spray-drying of the suspension, which now contains the prepared powder. The quantity of tin to be added depends very much on the combination of plastic material and solvent and their relative volumes and also on the method of drying of the powder. In so far as the plastic material can be completely dissolved in an organic solvent, a tin compound, also in an organic solvent, may be added. Then the prepared powder can be obtained by a conventional process of drying, e~g. by spray-drying of the plastic-containing solution.
Another method, which has been successfully used, is a chemical precipitation of an Sn compound on the powder particles during continuous stirring.
Example 3 Precipitation of a tin compound on each plastic particle.

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1 16~778 10 g of a plastic powder is suspended in 200 ml of methanol.
While stirring, a saturated solution of SnC12, 2H20 in methanol is added~ the quantity depending on the size of the powder particles. The weight of the SnC12, 2H20 may be, for example, 1-10% of that of the plastic powder. While stirring an alkaline agent is added, causing the suspended plastic powder to work as a scavenger for the precipitation of hydroxytin-chlorides. The powder prepared in this way can then then be dried, and if necessary ground and sieved.
By the development of the method, according to the invention, sensitization of an insulating surface has been obtained with a view to a subsequent electro-less metallization, performed by known methods, without demanding preceding special matters or masks~ as any pattern which can be copied by a xerographic process can be transferred to the desired substrate surface. The method does not put heavy demands on resources nor is it detrimental to the enviroment, as etching-off of the superfluous metal layer is avoided. Furthermore, the metal layer transferred by electroless metallization has a much better adherence to the surface of the substrate (e.g. as measured by the Scotch-Tape method) than layers of the same thickness, which have been transferred by conventional electroless deposition or by evaporation or vpd methods (vpd: vapour-phase deposition). In , J
., addition, the present method makes much less demands on -the cleaning of the surface of the substrate prior to metallization than do the methods rnentioned above.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for the dry, selective sensitization of the surface of an insulating substrate and subsequent electroless deposition of a metal, comprising: xerographic-ally depositing a dielectric powder containing a metal compound on an insulator substrate in a desired pattern;
fixing said deposited powder on the surface of said sub-strates; and electrolessly depositing a conductive metal from solution onto the pattern defined by said xerographic-ally deposited powder.

2. A method as claimed in claim 1, wherein said fixing comprises treating the substrate and the powder with an organic solvent to obtain adherence.

3. A method as claimed in claim 1, wherein said fixing comprises melting the substrate surface to obtain adherence between the substrate surface and the powder.

4. A method as claimed in claim 1, wherein the powder comprises a thermoplastic material with an Sn2+
compound precipitated on the surface of each powder particle.

5. A method as claimed in claim 1, wherein the powder comprises a thermoplastic material with an Sn2+
compound distributed in each powder particle.

6. A method as claimed in claim 1, wherein the powder is such that, when in a molten state, it wets the surface to be sensitized.

7. A method as claimed in claim 1, wherein the powder contains an infrared-absorbing material.

8. A method as claimed in claim 1, wherein the powder is a material with a large loss-angle at microwave frequencies.